Ammonia is used as a source gas in the chemical vapor deposition (CVD) of nitride films during the fabrication of semiconductor chips. Typical nitrides are silicon nitride, made by the reaction of silane and ammonia and titanium nitride, made by the reaction of titanium tetrachloride and ammonia. The presence of one to three thousand parts per billion (ppb) levels of moisture vapor from ammonia cylinders will result in a decrease of the performance properties of the nitride layer. Recently, new gallium nitride CVD technology has been shown to require even lower levels of moisture in the source ammonia than silicon and titanium nitride technology. The level of moisture must be reduced to below 200 parts per billion (ppb) to avoid performance problems.
Ammonia is currently supplied to electronics customers in cylinders with a specification of less than 3 parts per million (ppm) moisture. This “high” value of moisture was actually due to the past limitations of analytical technology and not the actual attainable levels. Because of this limitation, purification efforts could not be accurately certified.
To achieve 3 ppm or less moisture, higher moisture content ammonia from the source cylinder was transfilled in the vapor phase to the designated cylinder. This treatment was found to be sufficient to remove moisture levels down to the 3 ppm specification. However, it was found that a simple vapor phase transfill was not sufficient to reach ppb levels of moisture.
Commercially available point of use in-line purifiers can be used to guarantee moisture levels lower than 3 ppm. These purifiers use a lithium based resin to remove moisture from ammonia or a zirconium-iron catalyst to remove moisture. Recent analytical technologies have shown that these purifiers will remove moisture to ppb levels. These purifiers are very expensive (3 to 5 thousand dollars) and have limited moisture capacity. They are not regenerable and must be replaced when spent. Because of these limitations, point of use in-line purifiers can not be used for large-scale purification, on the order of thousands of pounds per day.
It would be desirable in the art to provide an economical method for purifying semiconductor gases of low moisture level (less than about 200 ppb).